The evolution of the Leak TL12 Point One amplifier from 1948 right up to today
- Tim Mellow
- Jul 3
- 6 min read
The evolution of the Leak TL12 Point One amplifier from 1948 to the present day is a fascinating journey through audio engineering, marked by innovation, adaptation, and a lasting legacy in high-fidelity (hi-fi) audio.
Our Design Lead Tim Mellow gives a concise overview of the TL12.1's development, addressing its historical significance, technical advancements, and its influence in both the development of Hifi Audio and also the English Acoustics flagship 41c Stereo Amplifier.
In the beginning
One of the myths of audio history is that the first amplifier design to boast 0.1% total harmonic distort at full power was the Williamson Amplifier published in Wireless World in April and May 1947. Leak had already been producing a similar design as far back as 1945 for large companies such as GEC that were destined for public address use because hi-fi as we know it did not exist at that time. As well as the Leak Point One Type 15 which used triode-connected KT66 valves, there was also the Point One Type 30 in which they were pentode-connected for greater power, and these were sold as the GEC type 2430A.
However, there was a problem. The Williamson circuit was expensive to manufacture. It had an input gain stage directly coupled to a cathodyne (concertina) phase splitter which fed a push-pull driver stage. This in turn fed the power output stage which comprised a pair of triode-connected KT66 beam tetrodes. Hence there were four stages in total.
The TL12 is born
In a moment of inspiration, Harold Leak came up with the idea of replacing the cathodyne phase splitter and push-pull driver stage with a long-tail pair. In the process, he replaced three L63 triodes with a single ECC33 twin triode. By eliminating one of the stages, he increased the stability margin when applying that all important negative feedback to reduce the distortion down to 0.1%. Thus, the Leak TL12 Point One was born and launched in December 1948 just as vinyl was about to make its first appearance in music stores. Because it was aimed at the domestic market, it marked the birth of the hi-fi amplifier.
Unlike Williamson, all the Leak amplifiers mentioned above were using pentode input stages to increase the gain and the TL12 Point One used the EF36. Just as the previous Leak amplifiers foreshadowed the Williamson circuit, the TL12 Point One will look very familiar to anyone who has seen the Mullard 5-valve 10-watt design that was published in Wireless World in August 1954! This demonstrates how far ahead of the curve Harold Leak was.
Highly Revered
Over the years, the TL12 Point One has developed something of a cult following, garnering prices in excess of £5000 for an unrestored working pair on the secondhand market.
Unfortunately, the Haddon output transformers have a reputation for failing due to a breakdown in their insulation. By the time Leak introduced the TL12 Plus a decade later, in which the triode connected KT66 was replaced by an ultra-linear connected EL84, they had switched to Hinchley transformers which can easily be identified by their distinctive fully shrouded end caps. These have proved to be far more reliable but the TL12 Plus has never been as revered as the Point One. The choice of the ECC81 for the long-tail pair is a strange one as it was never intended for audio and produces quite a lot of odd order harmonic distortion that isn’t cancelled in push pull, unlike the predominantly even order harmonic distortion that audio valves produce. In retrospect, the 5751 (B9A equivalent to the octal 6SL7) would have been a much better choice having the same gain but much better linearity. Unfortunately, the ECC81 was used across the Leak “Plus” range of monoblocks, but luckily not in any of their stereo valve amplifiers.
It was something of an honor, as well as slightly daunting, to be asked to replace the output transformers in a pair of TL12 Point Ones recently. Please see my series of you tube videos on the restoration of these TL12 Point One’s. Furthermore, it was no ordinary TL12, if there could ever be such a thing, but one commissioned by the BBC to go inside their monitor loudspeakers. Custom modifications included a balanced input transformer, a variable input attenuator, a heat shield to deflect the heat from the valves out through the back, and a slightly different valve line up with the EF36 and ECC33 replaced by an EF37A and 6SN7 respectively. Hence, this was a piece of audio history. We measured the performance before and after fitting our in-house designed transformer and can report that the bandwidth of our new transformer design comfortably exceeded that of the old.
The influence of the TL12 on English Acoustics
And that brings us up to the present day. Fortunately, the evolution of Harold Leak’s designs did not end with the demise of Leak. For years, Peter Farrow had been rebuilding Leak Stereo 20 amplifiers which he had a fondness for having acquired one at an early age. One day, he realized that he could build them from scratch. This made commercial sense because, like the TL12 Point One, the price of the Stereo 20 had been creeping up over the years. Despite all the new technologies that have come along such as transistors, monolithic integrated circuits and class D, the classic Leak designs have a sound that cannot be replicated because they use the most linear amplifying devices on the planet, namely valves, in simple circuits with relatively little feedback by today’s standards. It is rather akin to cooking with good quality fresh ingredients in simple but tasty recipes without artificial additives. By contrast, modern audio gear contains hundreds of transistors on monolithic integrated circuits and is more akin to ultra processed food. Also, these classic amplifiers have no built-in obsolescence as there is nothing in them that cannot be replaced, unlike modern products that contain chips which become obsolete within a few years. Hence, we coined the phrase “heirloom class audio”.
The English Acoustics Stereo 21c and 41c
Therefore, Peter founded English Acoustics and the Stereo 21c was born. Essentially this was a faithful reproduction of the Stereo 20 for the 21st century. The only difference was that it benefited from modern components and had many safety features that would only have been a dream when the original was launched at the London Audio Fair at the Waldorf Hotel in 1958 in time for vinyl to go from mono to stereo. Again, Harold Leak was ahead of the curve!
Following the glowing reviews of the Stereo 21c, we launched its bigger brother the Stereo 41c and now our flagship amplifier, which produced around twice the power for today’s relatively inefficient speakers. However, these valve amplifiers sound significantly louder than one would expect from their power rating because they limit much more gently than solid-state amplifiers at full power due to having less feedback. Music has a crest factor of around 20 dB which means that the peaks have one hundred times the power of the average level. As you turn up the volume on a valve amplifier, the peaks get squashed without producing that harsh buzzy solid-state distortion while the rest of the music continues to get louder. Hence, we simply end up with a form of dynamic compression.
In the 41c, we evolved the Leak circuit slightly by feeding some of the output from one half of the long-tail pair into the other half to improve the balance. Hence, we no longer need to use unequal anode load resistors. However, the evolution doesn’t stop there.
Last year, Tim Mellow and Peter Farrow published an article in audioXpress entitled “A low-feedback triode amplifier with zero output impedance”. Although many audiophiles like the sound of low-feedback amplifiers, they don’t like the soggy bass which results from the raised output impedance. The article describes a way to solve this by using a mixture of negative voltage feedback and positive current feedback to cancel the output impedance with only a small amount of overall negative feedback. We will talk about the detrimental effects of feedback in a future blog.
The future
In summary, we are excited about the future and look forward to bringing Harold Leak’s legacy to a new generation of listeners.
Obviously, hi-fi isn’t meant to replace live music, but it does enable us to enjoy the back catalogue of artists who no longer perform live and it can’t be denied that giving ourselves a break from staring at screens in order to relax to our favorite music is nothing but good for our wellbeing.
The English Acoustics Difference
We’ve refined the valve amplifier into an object of the highest performance, durability, and timeless design. Every one of our amplifiers is designed and handmade in England with military-spec components — built not just to sound beautiful, but to last.
Want to hear the difference? You really have to listen for yourself. Get in touch for a demo either privately or at one of our esteemed dealers.
You may own some TL12.1's or Leak Audio equipement. If you'd like them restored let us know.
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